[TASK] Remove leftovers of `lib.stdheader`

Since the removal of css_styled_content the TypoScript snippet
`lib.stdheader` is gone. This patch removes some leftovers in the
`addPItoST43` method and some other places.

Resolves: #98012
Related: #80876
Releases: main
Change-Id: I3e1b2795c05b464f4210abc0968f507f20f9fba7
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75273


Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Benni Mack <benni@typo3.org>